Diffusion and adsorption of uranyl ion in clays: Molecular dynamics study

Molecular dynamics (MD) simulations were performed to study the diffusion and adsorption behaviors of aqueous uranyl species in interlayer space of clay minerals, which are key processes in the safety assessment for geological disposal of radioactive waste. The diffusion behaviors of UO22+, K+, CO32− and Cl− and H2O were evaluated for the aqueous solution system. The diffusion coefficients of these species decreased with increasing solutes concentration, and that of UO22+ was much smaller than others. In the aqueous solution with higher concentration of carbonate ions, uranyl carbonate complexes such as UO2CO3 and UO2(CO3)22− could be observed. For the clay-aqueous solution system (clay: montmorillonite or illite), the adsorption and diffusion behaviors of UO22+ and K+ were evaluated by MD calculations. The distribution coefficients (KD) of these solutes increased with the layer charge of clay, and the KD value of UO22+ might be smaller than that of K+. Further, the diffusion coefficients in the adsorption layer on the basal plane were relatively small and decreased with increasing the layer charge of clay.